1. Field of the Invention
The invention relates to methods of transparentizing fibrous sheets such as paper and to the transparentized fibrous products of the method.
2. Brief Description of the Prior Art
Transparent papers and like fibrous sheets are widely used in the packaging, printing and engineering fields. The purposes for using transparentized fibrous sheets such as paper vary from a need for reduced visual and reprint opacity to needs for chemical and physical resistance to degradation under certain conditions of use.
Engineering and reproduction applications for transparentized papers require certain physical and chemical characteristics such as good pencil and ink receptivity for tracing media, and a receptivity combined with frictional resistance which overcomes uneven penetration of coating solutions used in reprographic papers. When these papers are used in drafting, visual brightness is required and high UV translucency is necessary for making good reproductions. In practice, each grade of transparentized paper serves only a particular application. These papers are not interchangeable for each particular application. It is also desirable that a transparentized paper have good physical strength to resist handling, good permanence for record keeping, and chemical stability. Components applied to render the paper transparent should not migrate or volatilize, since that would result in a loss of transparency and contamination of interfiled documents.
Paper is a fibrous material composed of a plurality of discrete fibers disposed in a sheet configuration with many voids between the fibers. The disposed fibers scatter incident light. The physical process of rendering paper transparent comprises filling the voids between the fibers with a material that has a refractive index identical to or very close to that of the paper fibers. It should be noted that mineral fillers in papers affect the transparency because of their light absorption and light scattering properties and that papers without mineral fillers are preferred for transparentizing use.
Transparentizing paper is a widely established art and has been previously accomplished by impregnating the paper with mineral oils or waxes, polymeric thermoplastic resins such as polystyrenes, polybutenes, polypropenes, various rosin derivatives, and various polymers or copolymers of acrylics and styrene, with or without plasticizers. The application of such materials to the paper is done by either hot melt applications or by applying organic solvent solutions or aqueous emulsions to the paper. The processing equipment used for applying or treating fibrous material such as paper, varies with the type of application employed. For example, the transparentizing mixture may be applied by extrusion, tub, or roller applications and excess material may be removed by a doctoring off process using scrapers, wire, squeeze rollers, or size press. Where paper is employed for transparentizing, it sometimes is "wet packed" after treatment to improve the distribution of the transparentizing agents in the body of the paper and then after a certain time the "wet packed" paper is passed through a dryer to remove the solvent vehicle.
Where the transparentized paper is to be employed for drafting use, the transparentizing treatment may be followed by a surface application with resins and toothing agents such as particles of silica.
There are many disadvantages and problems associated with the prior art processes for transparentizing fibrous materials. High molecular weight resins would be desirable for transparentizing fibrous materials because of their physical strength, heat and water resistance, and absence of tendency of the resin to migrate within or out of the finished fibrous material. Such resins, however, can be applied to the fibrous material only with great difficulty since they tend to accumulate near or adjacent the surface of the fibrous material and when impregnation is accomplished the resin is generally unevenly distributed throughout the fibrous material. Thus, the translucency of the product made therefrom is low and is not uniform.
Low molecular weight liquid resins of low viscosity or mineral oils readily penetrate the fibrous material and produce a material of high translucency, but the finished surface would be soft and would not be pencil resistant. The surface would ghost upon erasure. Also, the transparentizing agents tend to migrate from the paper to interfiled documents lying adjacent thereto. Further, such low molecular weight resins or low viscosity materials are volatile and cannot be used where the transparentizing fibrous material is later subjected to elevated temperature processing equipment, such as xerography machines. The surface of such transparentized material is often tacky to the touch and therefore is undesirable.
Polymeric transparentizing agents which contain polaric and non-polaric groups in the chain are wetting agents and will distribute more readily through the fibrous material. However, such agents form a transparentized fibrous material which is too soft to the touch. Further, such transparentizing agents are mobile and easily leach out in water. They are not set in the fibrous material and would not be suitable, for example, for the later application of a diazo-sensitizing solution because such coating would be allowed to deeply penetrate the paper.
Other transparentizing processes comprise polycondensation of polyols with polyisocyanates in situ in the paper. The resulting polyurethane impregnated sheets, however, have insufficient translucency for tracing applications and upon aging produce yellow discoloration, even if so-called non-yellowing polyisocyanates are used. Further, transparentizing processes with polyurethane forming condensation reactions in-situ generally require the presence of substantial concentrations of non-polaric solvents like xylene to prevent accumulation of residual water in the fibrous material as well as hydroxy groups of cellulosic fiber to interfere with the condensation reaction. The condensation reaction actually takes place in a solution phase and as the polyurethane product resins are still dissolved in the solvent, it is in reality not an "in-situ" process, but the resin sediments out of solution and on to the fibrous material only after evaporation of the solvent at elevated temperature in a separate converting step. It can be well assumed that the polycondensation reaction still continues during the solvent evaporation at elevated temperature. The resin thus is not intimately bound to the fibrous material nor homogeneously distributed and gives a low transparentizing effect.
Prior art "in-situ" polymerized fibrous materials undergo heat, photo or electron beam polymerization immediately after impregnation in a continuing web pass. In such cases, extreme polymerization conditions are mandatory for economic reasons in order to shorten the reaction time. The polymerization can be considerted a spontaneous one. Thus, obtained transparentized sheets are more or less stratified materials and have a resin film on the surface which is detrimental to tracing applications, reduces receptivity for diazotype coating solutions, and is a cause for curl problems. If in such applications spontaneous polymerization is avoided through milder conditions, polymerization is incomplete and monomers or dimers remain in the composition. This makes the sheet useless for various applications. Spontaneous polymerization can also result in brittleness of the polymer, causing breaking of the resulting paper sheet, under the pressure of a hard pencil.
Improved transparentized fibrous sheets overcoming many of the above-described shortcomings have been obtained through the process disclosed in U.S. Pat. No. 4,271,227. In this process, monomers with 3 ethylenically unsaturated chain radicals bonded to one carbon atom are distributed within the fibrous sheet material and thermopolymerized in-situ.
We have now found that further improvements in the performance of transparentized fibrous sheets and more flexibility in the manufacturing process for such sheets are obtained by the in-situ polymerization of monomers of certain acrylic and methacrylic hydroxy alkylene or polyhydroxy alkylene esters etherified with hydrous dicylopentene radicals.
The improvements over the prior art include higher translucency and an evener "look through" of the transparentized sheets. In the process of the invention, one may use a wider range of monomer concentrations without risk of blocking during the polymerization process. The monomers and polymers used in the process of the invention are also compatible with the transparentizing agents described in U.S. Pat. No. 4,271,227 and other conventional transparentizing resins such as polystyrenes, polybutenes, hydrogenated hydrocarbon resins and resin derivatives. Thus, the latter can be added in the process of the invention as modifiers or dilutents for certain applications.
We have also found that the transparentizing process of the present invention may be carried out advantageously by impregnation of a fibrous sheet with the monomers, winding up the impregnated material in a tight roll, at room temperature, and keeping the thus "wet packed" roll for a certain period of time at a temperature at which no polymerization takes place. Then, the temperature may be raised to initiate polymerization in a way like that of the process disclosed in U.S. Pat. No. 4,271,227.
We have also found that the polymerization of the monomer after impregnation of the fibrous sheet can be advantageously performed through photopolymerization techniques. The advantage is found in elimination of the need to heat the monomer impregnated material.